It's because the three branches or our government are co-equal

The legislative branch as represented by partisan gadfly (my favorite word today) Henry Waxman can't force the executive branch to comply to its fishing expidition request for information, no more than the executive can force Congress to divulge anyone that gave any background information to a congressional committee in helping craft a bill. The executive gets to consult with private citizens outside of government without who they are and what they said becoming public. Free flow of opinion and information. A private citizen will be hesitant to lend expertise to the government if they know it is public and the "other side" gets retribution in onerous tax law or trade, safety, or employment practice investigations. It makes no difference who they were. The plan itself is public.

Informationaly, Enron was an equal opportunity campaign donor. They had many more dealings and deals with the Clinton administration (8 years vs a little less than 1)than with the new Bush admisistration. The reason the Enron guys weren't called before the Senate committee (Dems in charge then) was because they quickly realized it would expose the Clinton Commerce Dept dealings on behalf of Enron. Not that it matters, but I would think Enron contributed little. They were not an energy company. They were an energy trading company.